Conventionally, a multiplicity of different step-up voltages for use with different load apparatuses are generated from a rechargeable battery.
FIG. 5 shows a conventional power supply circuit adapted to generate a multiplicity of step-up voltages from a single power source, as disclosed in Japanese Patent Application Laid Open No. 07-176698. The power supply circuit has step-up circuits 81-83 for generating three outputs Vo1-Vo3.
FIG. 6 shows another conventional power supply circuit having a step-up circuit 90, which is adapted to generate an output voltage Vo that corresponds to the highest one (Vo1) of the required output voltages and regulators (LDO) 91, 92, and 93 for reducing the voltage Vo to predetermined output voltages Vo1, Vo2, and Vo3.
Such conventional power supply unit as shown in FIG. 5, however, requires the same number of step-up circuits as the number of output voltages required. As a consequence, the power supply circuit, and hence the LSI that accommodates the power supply circuit, becomes large and results in increased costs.
Although the conventional power supply circuit of FIG. 6 requires only one step-up circuit 90, it generates a high step-up voltage Vo associated with the highest output voltage Vo1. Hence, there will be large differences between the highest output voltage Vo1 and other output voltages Vo2 and Vo3. This implies that the regulators entail large amount of energy loss that lowers the power conversion efficiency of the power supply circuit.